The present invention relates to real-image finders and, more particularly, to a real-image finder with an image-inverting optical system that is suitable for use in still cameras, still video systems, etc. in which a photographic optical system and a finder optical system are provided separately from each other.
In lens-shutter cameras and so forth, a photographic optical system and a finder optical system are provided separately from each other. Finder optical systems of this type may be roughly divided into virtual-image finders and real-image finders. Virtual-image finders have the disadvantage due to the arrangement thereof that the diameter of the front lens is unfavorably large and the visibility of the view frame is not good. Accordingly, virtual-image finders involve a serious problem in achieving compact and high-performance finder optical systems. In contrast, real-image finders have an arrangement in which a view frame is placed in the vicinity of an intermediate image plane of an objective optical system, and the view frame is observed through an ocular optical system. Therefore, the boundaries of the view frame can be seen clearly. Moreover, because the position of the entrance pupil is close to the object side thereof, the objective optical system can be reduced in size in the diametric direction. Therefore, most of the recent compact and high-performance lens-shutter cameras employ real-image finders.
Thus, real-image finders can be reduced in size in comparison to virtual-image finders. That is, the diameter of the entrance-side lens can be made smaller than in the case of virtual-image finders. However, a real-image finder consists of an objective optical system, an image-inverting optical system, and an ocular optical system. Therefore, the overall length is unfavorably long. When such a real-image finder is mounted in a camera, the thickness of the camera increases disadvantageously. Therefore, attempts to achieve a compact real-image finder are generally made by devising methods to effectively fold the image-inverting optical system, which includes a prism and a mirror.
Recent lens-shutter cameras equipped with zoom lenses are required to have both a high zoom ratio and a compact and high-performance structure. Many of these lens-shutter cameras employ a collapsible mount type photographic optical system to reduce the overall size of the camera when not used. That is, when the camera is not used, the taking lens is withdrawn into the camera body; when the user is going to photograph a picture, the taking lens is extended to an operative position. In the finder optical system, which is provided separately from the photographic optical system, the amount of change in the field angle required increases as the zoom ratio of the taking lens becomes higher. In general, however, the finder optical system itself is not allowed to project from the camera body. In addition, the camera body is becoming slimmer these days. Therefore, it is very difficult to achieve an even more compact structure and a higher zoom ratio at the same time in the present state of art. One of factors in preventing the real-image finder from becoming compact in the direction of the thickness is the power distribution in the objective optical system. In general, the real-image finder obtains an erect image by using an image-inverting optical system including a prism and a mirror. Therefore, it is necessary to ensure an optical path length sufficiently long to invert the image by the image-inverting optical system. Accordingly, the objective optical system generally adopts a retrofocus type power distribution in which the back focus is long relative to the focal length of the entire system. This type is unsuitable for reducing the overall length and increasing the zoom ratio. Accordingly, there have been made many propositions that a curved surface could be used to form a refracting surface of an image-inverting optical system using a prism to assign a part of the power of the objective optical system to the image-inverting optical system. In the present state of art, however, there is no solution to the problem of how to reduce the size of the objective optical system in the direction of the thickness and to increase the zoom ratio at the same time while ensuring a sufficiently long optical path length to effect image inversion in the image-inverting optical system.
Accordingly, there have recently been made some propositions that a curved surface be used to form not a refracting surface but a reflecting surface in an image-inverting optical system of a real-image finder. That is a reflecting surface of a prism or a mirror that constitutes the image-inverting optical system, could be curved thereby giving a power to the image-inverting optical system. Adopting such an arrangement makes it possible to minimize the back focus of the objective optical system while ensuring the optical path length required for the image inversion and hence possible to reduce the size of the objective optical system in the direction of the thickness. However, a reflecting surface of the image-inverting optical system is generally decentered with respect to the optical axis. If a power is given to the decentered reflecting surface, aberrations due to decentration that are rotationally asymmetric even on the optical axis are produced. The rotationally asymmetric decentration aberrations are basically impossible to correct by a rotationally symmetric surface.
U.S. Pat. Nos. 3,810,221 and 3,836,931 both disclose an example in which a rotationally symmetric aspherical mirror and a lens system having a surface which has only one plane of symmetry are used to form a finder optical system of a reflex camera. In this example, however, the surface having only one plane of symmetry is utilized for the purpose of correcting the tilt of a virtual image for observation. Furthermore, in this example, the taking lens of the camera and the finder optical system are not separate from each other.
Japanese Patent Application Unexamined Publication Number [hereinafter referred to as "JP(A)"] 8-248481 uses a rotationally symmetric curved surface as a reflecting surface of a prism that forms a real-image zoom finder of a lens-shutter camera. It is stated in the publication that an aspherical surface or a toric surface is applicable to the curved surface. However, the aspherical surface disclosed in the specification of the publication is rotationally symmetric. The toric surface is also symmetric with respect to two coordinate axes. Therefore, correction for skew rays cannot satisfactorily be performed. In either example, a curved surface is used as a reflecting surface of a prism. However, the prism does not have an image-inverting action but merely serves to ensure the required optical path length.
EP0722106A2 discloses the use of a rotationally asymmetric curved surface as a reflecting surface of a prism in a real-image finder of a fixed focal length lens-shutter camera in addition to the subject matter of the above-described JP(A) 8-248481. As stated in the specification of the publication, the prism serves the function of an objective lens and does not have an image-inverting action.
JP(A) 8-292368, 8-292371 and 8-292372 perform image inversion in fixed focal length and zoom image pickup apparatuses by using a prism optical system having a rotationally asymmetric surface, but show no example in which the disclosed arrangement is applied to a finder optical system. These conventional techniques have no intention of applying the above-described arrangement to a finder optical system.